Actually I found out the reason by myself. However I think it shouldn't be like that by default or at least it needs to be added to the documentation of mysql remote connections.
The reason is that previously , when i launched my older server machine, the user firstname.lastname@example.org was existing. From a month or so all the new instances don't have this user. I just created it through phpmyadmin which is working by default and now everything is fine.
Just wanted to flag this out.